IronKnees,
Yes, the short stroking was causing the over/under charges also...that's one thing I like about the Dillon 900...if you short-stroke it, you are in trouble and it lets you know..that way it's almost impossible to get in the predicament you got in.
About those splits at the case mouth: This really isn't a problem until they get down to the end of the crimp on all or almost all of the petals. Just keep on reloading them and don't worry about it. I shot trap in competition for many years and still do it for the practice and I can detect no difference in patterns between one with the splits and one without. When you get down to where you can't get a crimp that will hold, your pressure will be too far down down because of inconsistent crimps, and discard. How do I know it doesn't make any difference? Because the clays smoke just the same with or without splits.
And, for confirmation of this, you might want to look at the Hodgdon Shotshell Data Manual 1st Edition starting on page 16. Here they took a hull through 15 firings and noted the Muzzle Velocity, pressure and signs of deterioration for each shot. Shot 1 was at 1,143FPS and 11,100 PSI. Shot 15 was at 1,150FPS and 10,900 PSI. And, they experienced the splits at the case mouth just as you did fairly early on.
Take care,
Ka6otm